1. Field of the Invention
This invention relates to a method for making radial ply tires in a single stage and more particularly to a method for making radial ply tires of the type including bias angled breaker plies and a 0.degree. cap band and to the construction of an uncured tire as an intermediate article of manufacture.
2. Prior Art
It has been the goal of the tire industry for many years to improve both safety and the service life of pneumatic vehicle tires, with much attention being focused on such tire characteristics as tread wear, handling and road holding ability, especially during cornering. In this regard the desirability of producing tires with a requisite amount of stiffness has long been recognized.
It should be noted, by way of definition, that in this specification and the appended claims, the term "0.degree. cap band" is used to describe a tire construction in which the cap band cords are oriented either truly circumferentially of the tire or at an angle of substantially 0.degree., i.e., preferably less than 1.degree. but in any event no more than about 2.degree., to that direction. The term "low angle" is used to designate belts having a cord angle of up to 25.degree. to the crown centerline while the term "high angle" is used to designate bias belts having a cord angle above about 38.degree. to the crown centerline. The term "rubber" as used herein is intended to denote both natural and synthetic rubbers or rubber-like materials, and blends thereof, specific formulations of which for a wide variety of different tire applications are well known and need not be explicitly set forth herein. The term "substantially inextensible" is used herein to designate the essential characteristics of filamentary reinforcing cords, which may be made of metallic materials such as steel wire or the like or of non-metallic materials such as cotton, rayon, nylon, polyester, polyvinyl alcohol, glass fiber, Aramid or the like, of being able to withstand, without substantial elongation, the tensile stresses in the belt normally encountered in service. The term "radially outer" is used with reference to the surface of the building drum and refers to a vector extending outwardly away from that face. The term "soft stretch" tape is used herein to denote a tape having the ability to be longitudinally considerably extended by the straightening of its component cord or cords without the latter being stretched.
A major step in the direction of improved safety and service life of pneumatic vehicle tires has been the advent of the belted tire, wherein a circumferentially extending belt or belt-like structure is incorporated under the tread in the crown region of the tire carcass so as to introduce a degree of restraint into the deformability of the tread. In such tires, as is well known, the belt is made of one or more plies of generally inextensible reinforcing cords which, in any given ply, are parallel to each other and confined between ply-wide skim coats of rubber and may be oriented at a predetermined angle to the crown centerline or median equatorial plane of the tire. Such plies are generally provided in pairs, with the cords in paired plies being oriented at equal but opposite angles to said plane.
It has been found that the angle which the cords in the breaker belt make with the crown centerline influences the performance characteristics of the tire. Too small an angle will result in low lateral stiffness resulting in poor cornering properties of the tire. Too large an angle will result in low circumferential stiffness and poor tread wear. To remedy the problem of any inherent weakness caused by a large bias angle, resort has been had to positioning a substantially 0.degree. cap band between the breaker belt and the tread to stiffen the tire. The cords in the cap band serve to exert a stability and uniformity enhancing compressive stress on the underlying breaker belt.
Additionally, in belted radial ply carcass tire construction, failure often occurs at high speeds because of separation occurring in the shoulder zones of the tire where the edges of the belt plies are located. Such ply separations are due to the cord ends at the edges of the belt plies becoming detached from the surrounding rubber under the combined effects of centrifugal force acting on the tire, flexing of the tire and heat buildup in the tire. The ply separation is made even more likely by the fact that the cords in the belt plies, being disposed at an angle to the crown centerline of the tire by virtue of said plies being cut at an angle with respect to the longitudinal direction of the cords, have a natural tendency to spread apart at their cut ends. The edges of the belt thus constitute zones where the cut and free ends of the reinforcing elements, i.e., the cords, by friction and by cutting cause breaks both in the carcass plies and in the rubber of the tire.
Cap band structures have been employed also in an attempt to eliminate such separation.
There are certain manufacturing drawbacks and disadvantages which attend the manufacturing of a radial ply tire having a 0.degree. cap band. Principally, this has been due to the fact that, by virtue of the inextensibility of the cords in the cap bands conventionally used, the production of a radial tire having a 0.degree. cap band has generally required a two stage building operation, in which a beltless carcass is built on a relatively low diameter cylindrical drum and is then, either at the carcass building station or at a different station, first shaped into toroidal form on specially constructed radially expansible and axially collapsible drum before the belt and tread are applied. A representative illustration of the prior art 0.degree. cap band radial tire technology is set forth in U.S. Pat. No. 3,850,219. The manufacture of the belted tire disclosed therein requires the building of the carcass in a substantially flat or cylindrical form. The carcass is then shaped to its toroidal form, whereupon belt plies, the cap band in an unstressed condition, and the overlying tread rubber, which were combined in a separate stage, are then applied to the crown region of the shaped carcass to complete the raw tire. Such two stage tire building installations are both complex and expensive.
It is also known to manufacture radial tires in a single stage. A representative illustration of this technology is set forth in U.S. Pat. No. 3,558,389. In the method disclosed therein the belt cords are arranged in corrugated cord fabric plies which are bias cut, and these are so applied to the carcass that the belt cords are oriented at relatively high angles, i.e., between 40.degree. and 60.degree. to the crown centerline during the building operation and then, during the shaping of the tires in the press, pantograph down to some lower angles, i.e., less than 20.degree. to the crown centerline. Such a method requires a special design for the breaker belt and does not make provision for including a cap band in the tire. In U.S. Pat. No. 3,373,066 there is disclosed a method for making a radial ply tire in a single stage which contains breaker plies having cords lying at angles of between 25.degree. to 35.degree.. When the tire is expanded into its toroidal form, the cords pantograph by the aid of lubrication which is applied to the breaker plies. The problem with applying lubrication during the tire building operation is that it can affect adhesion properties and excess lubricant tends to cause surface blemishes.
It is possible to manufacture radial ply tires in a single stage process without a specially designed breaker belt or the use of lubrication; however, the cords in the breaker belt in the tire usually lie at a relatively high angle with respect to the crown centerline. It requires a significant tension force to displace the cords in the belt to a low angle in the absence of specially designed breaker belts or lubrication and a tension force of the magnitude required cannot be applied successfully in ordinary manufacturing operations. It has been found also that as one attempts to displace the cords to lower angles, tire uniformity problems are encountered as the cords have a tendency to move in a less uniform manner. It should be noted that the tire uniformity problem also exists when the cords lie initially, at a low angle and are pantographed to a still lower angle during shaping.
A tire having a breaker belt of cords oriented at a high bias angle has a low hoop modulus and a relatively low stiffness. Theoretically, the problem of a relatively low hoop modulus of the tire having high bias angled breaker ply cords can be dealt with by providing a substantially 0.degree. cap band.
It should be noted that while there is some expansion of the cap band during the second stage process this expansion is minimal when compared to the expansion which must be undertaken by the cords in the cap band in a single stage process. Since the cords comprising the cap band are already substantially at 0.degree. relative to the crown centerline of the tire carcass, there can be no slippage or movement toward a circumferential orientation as occurs with the cords in the breaker plies, and, because the cords of the cap band are generally inextensible, they cannot stretch to the degree necessary in a single stage process to accommodate the expansion of the carcass.
It is towards elimination of these and other problems that the present invention is directed.